A color picture tube includes a faceplate and a funnel which are integrally joined together. The inside surface of the faceplate is covered with a phosphor screen composed of triads of phosphor elements which emit the three primary colors of light, red, green and blue when impacted by electrons. An electron gun is mounted in a neck portion which is attached to the funnel in a position remote from the faceplate. The electron gun provides three electron beams which are used to scan the phosphor pixels to cause the desired image to be displayed. A shadow mask is arranged in the proximity of the phosphor screen and is used as a color selection electrode to assure that each of the three electron beams impacts the phosphor of the proper light emitting color. Thus, for example, the electron beam which is modulated with the red data impacts the phosphor pixels which emit red light. Because the electrons are charged particles, the Earth's magnetic field has an influence on their trajectories which can cause the electrons to impact a phosphor of the improper color, a phenomena known as misregistry. For this reason, a magnetic shield is commonly used, either in the interior or on the exterior, of the picture tube, to shield a substantial portion of the electron beam trajectories from the influence of the Earth's magnetic field. Most recent tubes utilize an interior magnetic shield (IMS) which is attached to the shadow mask and extends toward the electron gun.
The magnetic effect on electron beams, which causes misregistry, occurs in the directions which are perpendicular to the longitudinal axis of the tube. For this reason, various changes in the configuration, or structure, of the internal magnetic shield can beneficially influence the misregistration in one direction and adversely influence it in an orthogonal direction. Misregistry must be corrected in all three field directions: axial, horizontal, and vertical. The axial (north-south) field acts parallel to the longitudinal axis of the tube. The horizontal (east-west) and vertical fields act along the horizontal and vertical axes of the faceplate, respectively. In the early prior art, the vertical field was shielded from the interior of the tube by enclosing the interior of the tube as completely as possible. This entailed attaching an internal shield to the mask and minimizing the size of the opening facing the electron gun. In later prior art, the axial field was reshaped to have a vertical component by the formation of V-notches on the sides of the shield which enlarged the rear opening facing the electron gun but degraded the vertical-field shielding. Since the horizontal field is reshaped by the shadow mask, the shield generally interferes with this function by the shadow mask, the shield generally interferes with this function of the shadow mask. This interference is reduced in the prior art by placing vertical cuts in the shield to section it horizontally, e.g., by placing vertical slots along the minor axis of the shield. These cuts, or slots, further reduce the enclosure of the tube interior by the shield, thus further degrading the vertical-field shielding ability of the shield. Thus, the prior art is generally deficient in providing adequate shielding in all three fields. The present invention is directed to a tube having an internal magnetic shield which has a favorable influence on electron beam misregistry in all directions.